We need 1/2 cup of oil and we are told that we already have 1/3 cup of oil, to find out how much we need we subtract this and the result will be the amount of oil we need.
[tex]\frac{1}{2}-\frac{1}{3}=\frac{3-2}{6}=\frac{1}{6}[/tex]In conclusion, the answer is 1/3 cup of oil